1. Field of the Invention
The present invention relates to a voltage detection device and an equalization device. More particularly, the invention relates to a voltage detection device and an equalization device which detect a voltage across the both ends of each of plural unit batteries that are connected to each other in series.
2. Description of the Related Art
Battery assemblies mounted in, for example, hybrid vehicles and electric vehicles are composed of plural unit batteries that are connected to each other in series. A high voltage of 200 V, for example, appears between their both ends and generated power is supplied to a drive motor. In such battery assemblies, if a variation occurs between the voltages across the pairs of ends of the respective unit batteries, they may be reduced in the efficiency of use or charged excessively. In view of this, equalization devices have been proposed which detect a voltage across the both ends of each of the unit batteries and the voltages across the pairs of ends of the respective unit batteries are equalized on the basis of detection results using discharge resistors etc.
Such equalization devices are composed of battery monitoring ICs each of which incorporates, among other things, Analog-to-Digital converters for detecting voltages across the pairs of ends of the unit batteries, respectively. To increase the voltage detection accuracy of the battery monitoring ICs, a lowpass filter (hereinafter abbreviated as LPF) is provided between each unit battery and the associated Analog-to-Digital converter to cut high-frequency noise.
However, in the above conventional technique, when an LPF fails, it may be impossible to detect the LPF failure depending on its type. More specifically, in the case of a failure that the output voltage of an LPF decreases markedly in a short time due to, for example, breaking of a resistor of the LPF that results in its opening, the LPF failure can be detected on the basis of a detection result of the voltage across the both ends of the unit battery. On the other hand, in the case of a failure that the output of an LPF decreases gradually over a long time due to, for example, a leak failure of a capacitor, the LPF failure cannot be discriminated clearly from degradation of the characteristic of the unit battery itself and hence it is difficult to detect the LPF failure soon after its occurrence. Where an LPF failure cannot be detected promptly as in the latter case, a detection value of the unit battery is lower than an actual value until detection of the failure, possibly lowering the efficiency of use of the battery or causing its overcharging.
The technique disclosed in JP-A-2012-122856 has been proposed as a technique for detecting disconnection of an LPF. However, although the device disclosed in JP-A-2012-122856 can detect disconnection of an LPF, it cannot detect a leak failure of a capacitor.